This invention relates to a method and apparatus for detecting the surface height (melt level) of a raw-material melt in a Czochralski monocrystal pulling apparatus, and more particularly to a method and apparatus for detecting the melt level of molten silicon.
The Czochralski method (CZ method) is a method for pulling out monocrystalline ingots from melts of raw materials such as silicon in a crucible. In order to make crystals grow well, it is necessary to accurately detect the surface level (hereinafter xe2x80x9cmelt levelxe2x80x9d) of a raw-material melt and to adjust the same.
Detecting and adjusting melt levels properly in a CZ monocrystal pulling apparatus is useful in controlling the relative positions of a heat shield and the melt level, or the relative positions of a heater and the melt level so as to promote stable crystal growth.
In particular, in currently existing CZ silicon monocrystal pulling apparatuses, normally, the thermal radiation from the heater and silicon melt are controlled and also a heat shield (or gas rectifying tube) for rectifying a gas flowing inside the furnace is installed. By appropriately performing the feedback control described above, and controlling the relative positions of the lower surface of the heat shield and the melt level (that is, the distance between them), the thermal hysteresis of the pulled silicon monocrystal and the impurity concentrations (oxygen concentration, etc.) therein can be made constant.
In terms of conventional technology for melt level detection apparatuses, there are apparatuses such as that disclosed in Japanese Patent Publication (Kokoku) No. 3-17084. That conventional apparatus detects melt levels based on the principle of triangulation. More specifically, with that apparatus, as illustrated in FIG. 19 (FIG. 1 in this Publication), a laser beam 34 is projected onto the melt surface at an angle xcex8, the regularly reflected beam 38 is condensed by a lens 44, and the position of convergence 46 is detected by a photosensor 48. In this conventional apparatus, the enlarged projection 30 of the laser beam is received for the purpose of averaging the measurement variation caused by very small ripples 22 that develop in the surface of the melt surface 20.
However, there are other factors in the melt surface 3 that impair the flatness of that melt surface 3 besides the very small ripples noted above. Specifically, as illustrated in FIG. 2(b), in the portion of the melt surface 3 near a crystal 15, a meniscus 28 develops due to the surface tension near the growth surface of the crystal 15. Also, due to the rotation of the crucible 14 and the rotation of the crystal 15 being pulled, the surface becomes inclined in a parabolic shape across the entirety of the melt surface 3. As illustrated in FIG. 2(c), furthermore, when a gas rectifying tube 16 is brought close to the melt level 3, due to the discharge pressure of the inactive gas, there are cases where a depression is formed in the melt surface 3 near the bottom portion of the gas rectifying tube 16. Such inclinations as these in the melt surface 3 cause the direction of the regularly reflected beam from the laser beam used for measuring as noted earlier to be shifted (the inclination in the melt surface 3 being indicated by the angle xcexa8 in the figure), making effective reception of that beam difficult.
The above-described conventional discloses that the position where the photodetector unit is disposed is moved so as to catching the shift in the direction of the regularly reflected laser beam, caused by inclinations in the melt surface. The inclination xcexa8 in the melt surface 3 is closely related to the rotational speeds of the crucible 14 and of the crystal 15 and to the height of the gas rectifying tube 16 above the melt surface 3. Therefore, every time these pulling conditions are changed, the amount to move the position of the photodetector unit must be adjusted. Not only is that a troublesome task, but it is very difficult to precisely reproduce movement settings for the photodetector unit. When the movement adjustment is not done well, furthermore, it is possible that the regular reflected beam from the laser will not be able to be received and that melt surface detection will be disabled.
Furthermore, as illustrated in FIG. 20 (which corresponds to FIG. 3 in the Publication mentioned above, being a diagram illustrating changes in the regularly reflected laser beam relative to changes in the melt level), when the melt level changes (xcex94L) largely, the positional shift (xcex94Y) in the regularly reflected beam becomes large, whereupon it becomes necessary to shift the light detector greatly, by that measure. In conjunction therewith, moreover, installation constraints develop, such as the necessity of having an observation port 40 of a size that corresponds with that large shift, or the need to provide enough space for the light detector to be able to move.
Furthermore, because the melt surface 3 reflects light as does a mirror surface, another problem arises in that, as illustrated in FIG. 15(a), scattered light at the emission port 29 of the laser source 1 is reflected by the melt surface 3, which is picked up in the photosensor 7 as a ghost 30 (a phenomenon which occurs frequently under conditions where the regularly reflected beam 4 is received through a lens 5) resulting in a deterioration in position detection precision. In this connection, in the conventional technology noted above, a band-pass filter is employed which passes only laser beam wavelengths, as a measure for distinguishing between the radiated light from the melt surface 20 and the received laser beam with good contrast. However, the wavelength of the scattered light at the emission port 29 of the laser source 1 and the wavelength of the laser beam that is regularly reflected by the melt surface 3 are mutually identical, wherefore the picking up of the scattered light at the emission port 29 by the photosensor 7 cannot be avoided with a band-pass filter.
An object of the present invention, which was devised with the problems noted above in view, is to provide a melt level detection apparatus and melt level detection method wherewith melt levels can be detected simply and precisely.
In order to resolve such problems as those noted above, in the melt level detection apparatus and melt level detection method according to the present invention, use is made of the shape of the surface that regularly develops on the surface of the melt, causing that to function as a kind of reflecting body (that is, a kind of reflecting body for causing the projected laser beam to be accurately guided to a photodetector attached at a prescribed location), and thus performing melt level detection accurately. As means to that end, scanning is performed in the radial directions of the crucible to search out a position at which the projected laser beam will be accurately guided to the photodetector.
The present invention, furthermore, was devised on the basis of certain points of view, namely that, in a CZ furnace, due to the rotation of the crucible itself and/or the rotation of the crystal being pulled out, a swell develops in the shape of a concentric circle centered on the axis of rotation of the crystal being pulled out, which is a regularly occurring phenomenon, and that, because the cross-section of that swell is parabolic, if scanning is done in the radial directions of the crucible, a position will always be found at which the projected laser beam will be guided accurately to the photodetector.
More specifically, the present invention provides a melt level detection apparatus and method such as those described below.
(1) A melt level detection apparatus comprising a laser beam projector and a photodetector at prescribed positions in a CZ furnace, wherein the laser beam projector projects a laser beam emitted from onto a surface of a melt, the photodetector receives the laser beam reflected from a site where the laser beam is projected, and a level of the surface of the melt inside the CZ furnace is detected based on principle of triangulation, wherein a position at which the laser beam projector projects is moved in radial directions of a crucible inside the CZ furnace, whereby the laser beam reflected from the surface of the melt scans a projection position at which the laser beam is received by the photodetector, and projection position of the laser beam is set at the position, whereby the level of the surface of the melt is detected.
The term xe2x80x9cregularlyxe2x80x9d connotes a concept that of course embraces such things as a standing wave that exists more or less constantly under certain conditions, but also includes the assumption of a condition capable of existing as a stable condition when a prescribed certain time period is in view. In the embodiments of the present invention, that which is typical as a xe2x80x9cregularly occurring undulationxe2x80x9d is a stable concentric circular swell centered on the axis of pulling that is produced by the rotations of both the crystal being pulled out and the crucible. The reason for this is that, although these things behave as stable standing waves when a prescribed certain time period is in view, because such a surface condition changes moment by moment as the crystal is pulled up, in the embodiments of the present invention, it is more natural to suppose that xe2x80x9ca condition is assumed that can exist as a stable condition when some prescribed certain time period is in view.xe2x80x9d
(2) The melt level detection apparatus described above, further comprising first light path alteration means for altering path of advance of the laser beam emitted from the laser beam projector and projecting the laser beam onto the surface of the melt, and/or second light path alteration means for altering path of advance of the laser beam reflected from the surface of the melt and guiding the laser beam to the photodetector.
(3) The melt level detection apparatus described above, wherein alteration of position to projection by the laser beam projector is performed by the first and second light path alteration means.
(4) Any of the melt level detection apparatuses described above, further comprising a beam attenuating filter which blocks light of a prescribed light intensity or below of light received by the photodetector.
(5) Any of the melt level detection apparatus described above, further comprising an angle adjusting mechanism which adjusts projection angle of the laser beam projector.
(6) The melt level detection apparatus described above, wherein the scanning of the projection position by the laser beam projector is performed up to a bottom portion of a heat shielding member disposed inside the CZ furnace; and position of the bottom portion of the heat shielding member is calculated by receiving also the laser beam reflected from bottom portion of the heat shielding member with the photodetector. The xe2x80x9cheat shielding memberxe2x80x9d is an item that blocks the radiant heat from the surface of the melt, which may also have a function for rectifying the gas that is flown into the furnace.
(7) The melt level detection apparatuses according to any one of claims 1 to 6, wherein the photodetector comprises a two-dimensional photosensor which simultaneously detects two-dimensional positions of a measurement spot on an observation surface. The concept of a xe2x80x9ctwo-dimensional photosensor for simultaneously detecting the two-dimensional position of a measurement spot on an observation surfacexe2x80x9d includes both the case where the two-dimensional photosensor itself doubles as a distance measuring sensor, and the case where the two-dimensional photosensor is provided separately from a distance measuring sensor.
(8) A melt level detection method for detecting a level of a surface of a melt inside a CZ furnace by using a laser beam projector and photodetector disposed at prescribed positions in a CZ furnace, based on principle of triangulation, wherein a position at which the laser beam projector projects is moved in radial directions of a crucible inside the CZ furnace, whereby the laser beam reflected from the surface of the melt scans a projection position at which the laser beam is received by the photodetector; and projection position of the laser beam is set at the position; and the projection position of the laser beam is set at that position.
(9) The method described above, wherein a beam attenuating filter which blocks light of lower light intensity than light of the laser beam emitted from the laser beam projector and regularly reflected from the surface of the melt is provided to eliminate ghosts other than the regularly reflected beam.
(10) The method described above, wherein the scanning is performed up to a bottom portion of a heat shielding member disposed inside the CZ furnace; and portions of the heat shielding member is detected by difference in reflectivity between the bottom portion of the heat shielding member and the surface of the melt.
(11) The method described above, wherein the photodetector comprises a two-dimensional photosensor which simultaneously detects two-dimensional positions of measurement spots on an observation surface; and the melt level and two-dimensional positions of measurement spots on the heat shielding member are detected.
(12) The method described above, wherein the scanning for detecting the level of the surface of the melt inside the CZ furnace based on the principle of triangulation is either performed at regular times or occasionally.
(13) The method described above, wherein the scanning performed occasionally is for finding a position where reflected beam receiving conditions are favorable, the scanning being stopped while the light receiving conditions are favorable, the scanning being resumed when light receiving conditions deteriorate and search is continued until a position is reached where reflected beam receiving conditions are favorable.
The present invention, described as in the foregoing, may be expressed in general terms as follows. Since the method according to the present invention is one that applies the principle of triangulation to a practically-used liquid, it is clear that the present invention can be applied to any liquid whatsoever as long as it can bring into being a stationary condition on the surface under certain established conditions.
(14) In a method of detecting a level of a liquid surface by emitting a laser beam from a prescribed position relative to the liquid surface whose level is to be detected, and receiving the laser beam reflected from the liquid surface whose level is to be detected, at a prescribed position different from the first-mentioned prescribed position whereby the level of the liquid surface is detected based on principle of triangulation, a method of adjusting direction of advance of the laser beam reflected from the liquid surface by utilizing slope of an undulation produced regularly in the liquid surface whose level is to be detected. What is meant by xe2x80x9cutilizingxe2x80x9d is not limited to cases of simply adjusting the direction of reflection, but also includes application cases such as where light is condensed using a concave surface.
(15) A method of detecting a level of a surface by emitting a laser beam from a prescribed position relative to the surface whose level is to be detected, and receiving the laser beam reflected from the surface whose level is to be detected, at a prescribed position different from the first-mentioned prescribed position whereby the level of the surface is detected based on principle of triangulation, wherein the method is a method for detecting a level of a surface that is emitting light, and reception of the laser beam is selectively performed by using a laser beam having a light intensity stronger than a light intensity of the surface whose level is to be detected, while receiving the laser beam through a beam attenuating filter positioned at an energy level between the light intensity of the laser beam and the light intensity of the surface.
This method also is a general method for projecting a laser beam onto a light emitting surface and performing triangulation, and, so long as that surface emits light, there is no particular limitation on the type (material, product quality, and so forth) or state (that is, solid, liquid, etc.) thereof.